An optical waveguide type ring resonator is formed by disposing an optical ring waveguide adjacent to an input/output optical waveguide and optically coupling both waveguides with a directional coupler.
A ring resonator usable as a dispersion compensator is described in F. Horst et al., “Tunable Ring Resonator Dispersion Compensators Realized in High-Refractive-Index Contrast SiON Technology”, post deadline paper, European Conference on Optical Communications 2000. Since a group delay depends on a wavelength and a coupling coefficient between an input/output optical waveguide and an optical ring waveguide, a ring resonator can be used as a dispersion compensator by controlling the coupling coefficient so as to have dispersion characteristics of inverse symbol to chromatic dispersion in an optical fiber.
Furthermore, a ring resonator used for an add/drop optical filter is described in Senichi Suzuki et al., “Ring resonators using hybrid stacked waveguides”, The Institute of Electronics, Information and Communication Engineers, Autumn Conference, c-234, 1992. In this case, it is also utilized to control filter characteristics by controlling a coupling coefficient.
A relative refractive index difference Δn between a core and a clad of a single mode optical fiber (hereinafter, referred to SMF) is 0.3%, and a relative refractive index difference Δn of a silica optical waveguide capable of optically coupling with the SMF at a low loss of 0.3 dB or less is within a range of 0.3% to 0.75%. When the relative refractive index difference between an input/output optical waveguide and an optical ring waveguide is set within a range of 0.3% to 0.75%, a free spectrum range (hereinafter, referred to FSR) of the optical ring waveguide becomes a maximum of 6 GHz or so.
In the former paper, an optical ring waveguide of a relative refractive index difference Δn is set to 3.3% and a bend radius of 500 μm is formed to realize a FSR of 50 GHz. In the configuration described in the paper, a relative refractive index difference of an input/output waveguide is set to the same value with that of an optical ring waveguide. In the paper, to obtain a satisfactory optical coupling between the input/output optical waveguide having Δn of 3.3% and the SMF having Δn of 0.3%, disposing a mode converting optical fiber between them is proposed. It is reported that the coupling loss of one end face is reduced by 1.2 dB by disposing the mode converting optical fiber.
In the latter paper, by increasing Δn of the optical ring waveguide and decreasing Δn of the input/output optical waveguide, both wider FSR and connection with the optical fiber at low loss are realized. Specifically, by setting Δn of the input/output optical waveguide to 0.75% and Δn of the optical ring waveguide to 2%, a FSR of 21.6 GHz is realized without a mode converting optical fiber.
In the configuration described in the former paper, a mode converting optical fiber is required, and therefore the number of components increases making the circuit size larger. In addition, although the coupling efficiency is improved, the coupling loss is still as high as 1.2 dB.
According to the configuration in the latter paper, when Δn of the optical ring waveguide is increased and Δn of the input/output optical waveguide is decreased, the optical coupling efficiency between the SMF and the input/output optical waveguide improves, but the optical coupling efficiency between the input/output optical waveguide and the optical ring waveguide deteriorates. In the latter paper, it is described that the optical coupling efficiency between the input/output optical waveguide and the optical ring waveguide can be low. However, when it is used for dispersion compensation such that described in the former paper, the optical coupling efficiency between the input/output optical waveguide and the optical ring waveguide is sometimes required to be 70% to 100%. The configuration in the latter paper cannot be used for such use.
By increasing Δn of the optical ring waveguide, basically, a circulation length can be shortened and as a result the FSR is extended. However, when Δn of the optical ring waveguide is increased, the optical coupling efficiency between the input/output optical waveguide and the optical ring waveguide or between the SMF and the input/output optical waveguide deteriorates.